The present invention relates to a rubber composition for use in one-piece golf balls and, for example, the cores of solid golf balls such as two-piece golf balls and multi-piece golf balls. More specifically, the invention relates to a rubber composition which, in a molded and crosslinked form, has a suitable hardness and a good resilience, and is thus ideal as a golf ball material.
One-piece golf balls, and the solid cores of two-piece golf balls and multi-piece golf balls which are composed of a solid core encased, either directly or over an intervening intermediate layer, by a cover, are generally obtained by vulcanizing a rubber composition containing as the base material a rubber component such as polybutadiene and containing also, for example, an unsaturated carboxylic acid metal salt such as zinc acrylate and an organic peroxide. The unsaturated carboxylic acid metal salt serves primarily as a co-crosslinking agent or a crosslinking aid in the rubber composition, and is known to have a large influence on the crosslink structure and crosslink density of the rubber.
Peroxide crosslinking is employed to crosslink rubber, and is carried out using one or more organic peroxide. The field of golf balls contains much recent prior art that employs two or more organic peroxides by making use of the difference in the decomposition temperatures of the organic peroxides. A variety of research exists on such peroxide crosslinking.
In addition to these organic peroxides and unsaturated carboxylic acid metal salts, various types of additives, such as antioxidants, sulfur, organosulfur compounds, inert fillers and zinc stearate, are also suitably included in such rubber compositions for golf balls in order to improve the physical properties and characteristics thereof.
However, golf ball users are highly demanding and so further improvements in performance are desired. There is an especially strong desire for improvements in rebound, which directly affects the flight performance.
Prior-art references which relate to the present invention include the following.    Patent Document 1: JP-A 2002-355339    Patent Document 2: JP-A 2004-041734    Patent Document 3: JP-A 2004-043714    Patent Document 4: JP-A 2009-120634